PROTOZOA 297 



affect all the cells. In the very first segmentations of 

 the ovum, in which none of these parts come into play, 

 but are equally distributed among the daughter- cells, 

 certain cells were separated that retained all their 

 rudimentary parts, and therefore the power to build 

 up a complete organism. These were the germ-cells. 

 While the great mass of the cells differentiated more 

 and more in the course of development, these remained 

 inactive, or only divided in such a way that all the 

 rudimentary parts were retained in their daughter- 

 cells. Hence the germ-cells persist through the whole 

 embryonic development, and only come into prominence 

 when it is over, and the animal is fully formed. The 

 inactive basic parts that they have all retained then 

 enable them to produce a new organism under proper 

 conditions. 



Thus the multicellular animals multiply by means of 

 germ-cells. How has this method of propagation arisen 

 out of that of the protozoa ? 



In the protozoa the single cell is also the germ-cell, 

 as it has to undertake all the vital functions. Every 

 cleavage of the unicellular being is a reproduction. 



There is a certain animal in our fresh waters called 

 the "pandorina." It consists of sixteen cells, all 

 homogeneous, and each of them discharging all the 

 functions. Each of the sixteen can reproduce the 

 animal, by detaching itself from the cluster and sub- 

 dividing until it makes sixteen cells. Hence in the 

 pandorina each cell can act as a germ-cell. There is 

 no division of labour. 



The next step brings us to the "vol vox," a green 



